Demand for mobile, portable and stationary personal communication continues to increase as these communication modes become easier to use and more widely available, as they offer a greater variety of services, and as their benefits become more visible. As a result of the increasing demand for a limited number of radio spectrum allocations, new multiplexing techniques that will increase spectral efficiency over conventional multiplexing techniques have been widely sought.
Conventional multiplexing techniques rely solely on frequency, time, or code division to allow multiple users in the same locale to communicate simultaneously with the base station in that locale. These techniques include frequency division multiplexing (FDM)--also called frequency division multiple access (FDMA); time division multiplexing (TDM)--also called time division multiple access (TDMA); and code division multiplexing (CDM)--also called code division multiple access (CDMA).
For FDM, each user's signals occupy separate frequency bands (one for transmit and one for receive) and no other user is assigned to those bands. FDM alone offers very restricted opportunities for increasing spectral efficiency, namely different spectrally efficient modulation schemes, but the potential of such schemes is limited by the severity of the radio communications environment and, even in the most benign environment, the maximum capacity is relatively low.
For TDM, all users occupy the same frequency band, and each is allocated a time slot within which transmission and reception can occur. The frequency bandwidth of each user's signal can be much greater than that in FDM, but a synchronized access scheme is needed to prevent multiple users' signals from being active simultaneously. TDM alone offers no increase in spectral efficiency and requires a complicated access protocol. The inactivity time in typical speech is already exploited in many speech compression methods, so this redundancy has already been used up.
For CDM (using direct-sequence spread spectrum signals), all users' signals occupy the same band and can be active simultaneously. Each user is assigned a unique spreading code which is used at the receiver to separate a desired user's signal from the rest. CDM offers some limited increases in spectral efficiency but requires power control methods that can be difficult to implement in severe fading environments. There can also be some burden in managing the distinct codes that are assigned to the users.
As a result, space division multiplexing (SDM)--also called space division multiple access (SDMA)--was developed to employ spatial filtering to separate spectrally overlapping signals from different users. For SDM, a system can separate a desired user's signal from the rest if its spatial characteristics (e.g., direction of arrival) are sufficiently different from those of the other users. SDM can multiply spectral efficiency by a factor equal to the number of spatially separable channels sharing a spectral band. Since this number can be roughly equal to the number of elements in the antenna array (which can practically be on the order of 100 depending on physical and/or cost limitations at the base station), potentially large increases in spectral efficiency are possible. Transmission power can be reduced, thereby reducing the interference level for other users and increasing mobile (or portable) battery life. Also, reduction or elimination of multipath fading can improve the received signal quality. However, except for variations of these techniques that use fixed multibeam or multisector antennas to further increase capacity, none of them fully exploits the multiplicity of spatial channels that arises because each user occupies a unique spatial location. SDM techniques adapt the antenna array either by estimating the directions of arrival of the spectrally overlapping signals and then using these estimates to compute appropriate weights for the spatial filter, or by minimizing the time-averaged squared error between a known training signal and the output of the spatial filter.
Several methods for adaptively adjusting spatial filters based on antenna arrays have been heretofore developed.
For example, in the "known reference signal" approach, a reference signal is transmitted in addition to the message signal. As a result, channel capacity for the message is reduced, especially as the severity of the environment increases the need for adaptation and as the number of elements in the antenna array is increased. When TDM of reference and message signal is used, receiver complexity is increased, some start-up overhead is incurred in assigning training codes, and signal bandwidth must be increased to compensate for reduced message capacity.
With the "time redundancy" approach, each user has a unique message block length (which is an extra complication), and a given block is transmitted twice (which reduces effective capacity by 50 percent).
With the "frequency redundancy" approach, the message is transmitted at two different carrier frequencies, which reduces effective capacity by 50 percent.
With the "DF-based beamforming" approach, for moderately to widely separated multipath reflections, and for a large number of sources in a frequency band (which is desirable for increased spectral efficiency), direction-finding (DF) based methods are impractical or unusable and incur prohibitive computational expense. Array calibration problems also arise.
It can be seen therefore, that SDM versions based on direction estimation have numerous disadvantages, including computationally intensive algorithms, poor performance in the presence of multipath signals arriving from different directions, the need to measure, store, and update array calibration data, and considerable sensitivity to errors in the array calibration data. Versions that require a training signal have different disadvantages, including the need to use spectral capacity to periodically transmit the training signal, the need to synchronize the received and locally generated copies of the training signal, and the need to adaptively increase or decrease the duration of the training signal to accommodate varying levels of interference.
Therefore, there is a need for a spatial filtering method of multiplexing communications signals which overcomes the foregoing deficiencies. The present invention fulfills that need.